Effects of vegetation control on ecosystem water use efficiency within and among four grassland ecosystems in China

Abstract

Through 2-3-year (2003)(2004)(2005) continuous eddy covariance measurements of carbon dioxide and water vapor fluxes, we examined the seasonal, inter-annual, and interecosystem variations in the ecosystem-level water use efficiency (WUE, defined as the ratio of gross primary production, GPP, to evapotranspiration, ET) at four Chinese grassland ecosystems in the Qinghai-Tibet Plateau and North China. Representing the most prevalent grassland types in China, the four ecosystems are an alpine swamp meadow ecosystem, an alpine shrub-meadow ecosystem, an alpine meadow-steppe ecosystem, and a temperate steppe ecosystem, which illustrate a water availability gradient and thus provide us an opportunity to quantify environmental and biological controls on ecosystem WUE at different spatiotemporal scales. Seasonally, WUE tracked closely with GPP at the four ecosystems, being low at the beginning and the end of the growing seasons and high during the active periods of plant growth. Such consistent correspondence between WUE and GPP suggested that photosynthetic processes were the dominant regulator of the seasonal variations in WUE. Further investigation indicated that the regulations were mainly due to the effect of leaf area index (LAI) on carbon assimilation and on the ratio of transpiration to ET (T/ET). Besides, except for the swamp meadow, LAI also controlled the year-to-year and site-to-site variations in WUE in the same way, resulting in the years or sites with high productivity being accompanied by high WUE. The general good correlation between LAI and ecosystem WUE indicates that it may be possible to predict grassland ecosystem WUE simply with LAI. Our results also imply that climate change-induced shifts in vegetation structure, and consequently LAI may have a significant impact on the relationship between ecosystem carbon and water cycles in grasslands.

Clipping alters the response of biomass production to experimental warming: A case study in an alpine meadow on the Tibetan Plateau, China

“…The significant seasonal changes of GPP and AGB (Table 1) were similar to those in previous studies and the average GPP and AGB values ( Figure 1) were close to previous observations in alpine meadows on the Tibetan Plateau (Hu et al 2008;Fu et al 2013b). …”

Construction and progress of Chinese terrestrial ecosystem carbon, nitrogen and water fluxes coordinated observation

“…Annual mean air temperature and annual precipitation were found to be the dominant factors influencing the spatial variation of WUE among forests , but leaf area index (LAI) was found to determine the spatial variation of WUE among grasslands (Hu et al, 2008). Further analysis firstly illustrated the close relationship between WUE and altitude among different ecosystem types and found the spatial variation of WUE can be comprehensively reflected by elevation and LAI .…”

“…In addition, based on network eddy covariance measurements, the spatial variations of WUE among forests , grasslands (Hu et al, 2008), and ecosystems having different ecosystem types were investigated, which were found to be differed among ecosystem types. Annual mean air temperature and annual precipitation were found to be the dominant factors influencing the spatial variation of WUE among forests , but leaf area index (LAI) was found to determine the spatial variation of WUE among grasslands (Hu et al, 2008).…”

“…Eddy covariance technique, which simultaneously measures carbon and water fluxes, laid an important data basis for analyzing the variations of WUE. Using eddy covariance measured GPP and ET, ChinaFLUX analyzed the dynamic of WUE in typical forests , grasslands (Hu et al, 2008) and cropland (Zhao et al, 2007) and found the "coupling and decoupling" phenomena in the dynamics of carbon and water fluxes. Then the biological and environmental mechanisms underlying the WUE dynamics , Hu et al, 2008 and their difference among regions were clarified .…”

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“…169 The climate change-induced shifts in vegetation structure, and consequently in leaf functional traits, such as leaf 170 area index (LAI) and specific leaf area (SLA), likely affected both water and carbon cycles of grassland communities 171 (Hu et al 2008b). In this study, we scanned 20-30 fresh mature leaves for each species at our plots using a leaf area 172 meter (AM200; ADC Bio-Scientific Limited, Herts, UK).…”